Method for obtaining extreme selectivity of metal nitrides and metal oxides

ABSTRACT

Methods for etching metal nitrides and metal oxides include using ultradilute HF solutions and buffered, low-pH HF solutions containing a minimal amount of the hydrofluoric acid species H 2 F 2 . The etchant can be used to selectively remove metal nitride layers relative to doped or undoped oxides, tungsten, polysilicon, and titanium nitride. A method is provided for producing an isolated capacitor, which can be used in a dynamic random access memory cell array, on a substrate using sacrificial layers selectively removed to expose outer surfaces of the bottom electrode.

RELATED APPLICATION INFORMATION

This application claims priority under 35 U.S.C. §119(e) to U.S.Provisional Patent Application No. 60/606,836, filed Sep. 1, 2004, whichis hereby incorporated by reference in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to fabrication of electronic devices and,in particular, relates to etching layers in the fabrication ofintegrated circuits.

2. Description of the Related Art

Since the introduction of the digital computer, electronic storagedevices have been a vital resource for the retention of data.Conventional semiconductor electronic storage devices typicallyincorporate capacitor and transistor type structures, which are referredto as Dynamic Random Access Memory (DRAM), that temporarily store databased on the charged state of the capacitor structure. In general, thistype of semiconductor Random Access Memory (RAM) often requires denselypacked capacitor structures that are easily accessible for electricalinterconnection. Many of these capacitor structures are fabricated withlayers of material including semiconductor, dielectric, and metal.

One highly effective capacitor structure is the double-sided containercapacitor. Such a device is described in U.S. Pat. No. 6,451,661 toDeBoer et al. Double-sided container capacitors typically have acup-shaped lower electrode. A capacitor dielectric is applied over thelower electrode both internally and externally to the cup-shaped lowerelectrode. The upper electrode is then deposited over the capacitordielectric. By employing this folding shape, the capacitor uses arelatively small amount of real estate on the integrated circuit butprovides a suitably high capacitance.

Some capacitor structures have lower electrodes that are fabricated byfirst forming sacrificial spacers within a recess in a substrate, andthen forming the lower electrode within the recess and between thesacrificial layer on the sidewalls of the recess. Such a method isdescribed in co-owned U.S. patent application Ser. No. 10/714,115, whichwas filed on Nov. 13, 2003. The selection of the materials used for thelower electrode material is often dictated by the availability of anetching process with a suitably high selectivity ratio between the twomaterials. The selectivity ratio is defined as the difference of theremoval rate between two materials during an etching process. Typically,an etching process is considered selective if the selectivity ratio isgreater than 10 to 1, but for many applications, that selectivity ratiois not sufficient. The sacrificial spacer must be etched away withoutsignificantly damaging the surrounding layers, especially the lowerelectrode. In addition to DRAM, other memory arrays, such asferroelectric and magnetic RAM, require separation of cells from oneanother.

There are two major methods of etching integrated circuits (ICs). First,dry etching involves the use of plasma processes to remove films fromthe substrate. While this method is usually very accurate, it isexpensive and can cause damage to underlying layers, as it typically haspoor selectivity due to a significant physical component to the etch.Wet etching involves using chemicals usually in an aqueous solution withetching chemicals such as hydrofluoric acid, nitric acid, and aceticacid. While wet etching is isotropic, meaning that it etches in alldirections equally, this process is more cost-effective, allows batchprocessing of wafers, and is typically more selective than dry etching.

Hydrofluoric (HF) acid chemistry solutions are used to etch many films,primarily silicon oxide. Although they can be slower, dilute HFchemistries are often used to control the etching process for otherfilms. Generally, a HF solution is considered to be dilute if theconcentration is less than about 100:1. For example, aluminum nitride(AlN) is etched by a solution of 500 parts water to 1 part HF at a rateof about 60 Å per minute. Hafnium nitride (HfN) is etched by the samesolution at a rate of about 90 Å per minute. Standardborophosphosilicate glass (BPSG) etches at about 30 Å per minute in thissolution. Thus, for example, when using the above solution, the rate ofselectivity between HfN and BPSG would be about 3:1.

Many etchants are not sufficiently selective for use in forming DRAMcapacitors, or they only allow for a limited selection of materials tobe used as the sacrificial spacer and lower electrode. Differentetchants must be used for capacitor structures with different electrodesand spacers. This adds costs and inefficiencies to the fabricationprocess. Improved selectivity of etch is similarly desirable in avariety of other contexts for semiconductor fabrication. Hence, therecurrently exists a need to reduce manufacturing costs associated withfabricating capacitor structures by simplifying inefficient procedures.To this end, there also exists a need to increase fabrication efficiencyby improving the processing techniques associated with fabricatingcapacitor structures.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a method is provided forforming an isolated capacitor on a substrate. The method comprisesforming a recess, which has a base and side walls, in a structurallayer. The recess is then lined with a metal oxide or metal nitridesacrificial layer. A first conductive layer is then formed within therecess over the sacrificial layer. The sacrificial layer is then removedfrom the recess using a reduced-H₂F₂ HF chemistry solution. A conformalcapacitor dielectric is formed over the lower electrode. A secondconductive layer is formed over the conformal capacitor dielectric.

In another aspect of the invention, a method is provided for etchingmetal nitrides and metal oxides. An aqueous hydrofluoric acid etchsolution having a ratio of water to hydrofluoric acid of between about1,000:1 to 10,000:1 by volume is applied to a metallic layer adjacent toan exposed silicon oxide based layer. The metallic layer is etched withan etch rate selectivity to the adjacent silicon oxide based layer ofgreater than about 500:1.

In another aspect of the invention, a method is provided for etchingmetal nitrides and metal oxides. The method comprises heating asubstrate that has a layer of a metal oxide or metal nitride. Abuffered, wet hydrofluoric acid etch solution with a pH of between about3.5 and 5.5 to the substrate. The solution etches the metal oxide ormetal nitride layer on the substrate with an etch rate selectivity to anexposed layer of silicon of greater than or equal to about 2000:1. Inanother aspect of the invention, a method is provided for forming anisolated conductor on a substrate comprising forming a recess on thesubstrate and lining the recess with a metal nitride or metal oxidesacrificial layer. An electrode material is deposited within the recessbefore removing the sacrificial layer with an aqueous etchant, whereinthe etchant has an etch rate selectivity to the electrode material ofgreater than about 500:1.

In accordance with another aspect of the invention, a capacitor isprovided in an integrated circuit. The circuit includes a conductiveplug, and a container-shaped lower electrode comprising titanium nitrideor tungsten directly connected to the conductive plug. A conformalcapacitor dielectric extends over inner and outer surfaces of the lowerelectrode. An upper electrode extends over the conformal capacitordielectric.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1I are a series of schematic cross sections, illustrating oneembodiment of a method for forming a capacitor structure in accordancewith a preferred embodiment of the present invention.

FIGS. 2A-2B illustrate one embodiment of a method using an etchant ofthe present teachings to selectively etch a metallic layer while leavingexposed oxide layers substantially unetched.

FIG. 3 is a flow chart of a method for forming a capacitor structure inaccordance with a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Methods are provided herein for selectively etching metallic layers,particularly metal oxides and metal nitrides. In the illustratedembodiments, a layer of metal oxide or metal nitride can be selectivelyetched relative to layers of doped oxides, undoped oxides, polysilicon,titanium nitride or tungsten. A layer or multiple layers of a metalnitride or a metal oxide sacrificial spacer is formed within layers ofdoped oxides, undoped oxides, polysilicon, titanium nitride or tungsten.The sacrificial spacer is then etched by using an etchant with a lowpercentage of the H₂F₂ species of hydrogen fluoride, creating a voidwithin the structure.

A method for forming an isolated capacitor on a surface is taught inanother embodiment, represented by the flow chart of FIG. 3. A recess isformed 210 within a structural layer. A sacrificial layer is deposited220 along the surface of the structural layer and into the recess. Thesacrificial layer is preferably metallic, and in the illustratedembodiment comprises a metal oxide or metal nitride. A first conductivelayer is formed 230 within the recess. The sacrificial layer is removed240 from the recess by using a highly selective etchant, preferably ahydrofluoric acid based etchant with a low percentage of the H₂F₂species of hydrogen fluoride. A capacitor dielectric is then formed 250conformally over the lower electrode. An upper electrode is then formed260 over the capacitor dielectric.

Hydrofluoric acid (HF) chemistries are often used to etch metal nitridesand metal oxides. There are several species in fluorinated chemistryetchants that accomplish etching. These include HF, H₂F₂, HF₂ ⁻, and F⁻.However, without being bound by theory, it appears that the species thatprimarily etches doped and undoped oxides is H₂F₂. Several species offluorides etch most metal nitrides and metal oxides, primarily H₂F₂, HF₂⁻, and F⁻. In one embodiment of the invention, the H₂F₂ species isreduced, and the HF₂ ⁻ and F⁻ species are increased in the solution. Inembodiments of the invention this is accomplished by extreme dilutionand/or buffering methods. In a preferred embodiment the H₂F₂ species iskept low relative to the four etching species, HF, H₂F₂, HF₂ ⁻, and F⁻.

The solutions with a low concentration of the H₂F₂ species are then usedto selectively remove metallic films, including metal oxide and metalnitride films. The etchant can also be used to pull back a metal oxidefilm when it is contained in a sandwich-like stack or to etch out thesefilms as sacrificial spacers where selectivity is needed relative tooxides, polysilicon, tungsten, and titanium nitride.

In one embodiment, the etchant is a highly buffered HF solution, raisingthe pH. Adding a base to the HF solution is one way to raise the pH.Many materials can be used to buffer the HF solution and raise the pH.At low pH, the solution has a significant concentration of H₂F₂.Preferably, the pH of the buffered solution is between about 3.5 and5.5. More preferably, the pH of the buffered solution is between about4.0 and 5.0. Most preferably, the pH of the buffered solution is betweenabout 4.25 and 4.75. One preferred embodiment of a buffered HF solutionis to use buffered oxide etch (BOE). Traditionally, BOE is used to etchoxides, but at very high buffer (e.g., NH₄F) concentrations, BOE doesnot substantially etch oxides. In a preferred embodiment, the buffer toHF ratio is greater than 400 to 1. In a more preferred embodiment, thebuffer to HF ratio is greater than 500 to 1. The etchant is not limitedto these preferred embodiments, as skilled practitioners will appreciatein view of the present disclosure that other methods of reducing theH₂F₂ species of hydrofluoric acid can be used.

In another preferred embodiment, the etchant is an ultra dilute HFsolution. Preferably, the ratio by volume of water to HF is betweenabout 1,000 to 1 and 10,000 to 1 as-mixed. More preferably, the as-mixedvolume ratio of water to HF is between about 1,500 to 1 and 5,000 to 1.Most preferably, the as-mixed volume ratio of water to HF is betweenabout 2,500 to 1 and 4,000 to 1. In one embodiment, approximately 1 mLof 49% HF was added to between about 1000 mL to 5000 mL of distilledwater. To accelerate the etch process, the solution is then heated to atemperature of between about 50° C. and 90° C., more preferably betweenabout 60° C. and 85° C. Within this range, the primary active species inthe solution is F⁻. A preferred pH range for the ultradilute HF solutionis between about 2.5 and 3.0. Using dilute HF, the mole fraction of H₂F₂can be reduced below 0.06 mole fraction, more preferably below 0.04 molefraction. In other words, preferably less than one of twenty of thehydrofluoric acid molecules will be H₂F₂. Table 1 below providesestimated mole fractions for each of the species.

TABLE 1 Estimated actual mole fractions for three as-mixedconcentrations Species\ Concentration 1500:1 by volume 2000:1 by volume5000:1 by volume HF .75 mole fraction .75 mole fraction .65 molefraction H₂F₂ .03 mole fraction .03 mole fraction .02 mole fraction HF₂⁻ .03 mole fraction .025 mole .02 mole fraction fraction F⁻ .18 molefraction .2 mole fraction .3 mole fraction

Using the above-described solutions, etch rate selectivity ratios areexcellent. For metal nitride etching, selectivity ratios to doped andundoped oxides can easily exceed about 500:1. For preferred materials tobe etched, such as hafnium nitride (HfN) and aluminum nitride (AlN), thesame ratio is greater than about 1,000:1. The selectivity ratio of metalnitride etch rates to polysilicon etch rates is even higher. For metalnitrides, the etch rate to poly is preferably greater than about2,000:1. For preferred materials HfN and AlN, the same ratio is morepreferably greater than about 9,000:1.

In a preferred embodiment, the sacrificial layer is Al₂O₃. To accomplishthe removal of the sacrificial layer, the etching solution should getall the way down a narrow capillary where the sacrificial layer resideswithout excessive damage to surrounding materials, as will be betterunderstood from the discussion of the figures below. The removal of thesacrificial layer should accordingly be quick. For a metal oxide, a rateof about 5000 Å/min or faster is desirable to etch to the bottom of thecapillary where the sacrificial layer resides. When using certainmaterials, such as Al₂O₃, as the sacrificial layer, heat is preferablyused to expedite the etch process. The solution is preferably heated toa temperature of between about 50° C. and 90° C., more preferablybetween about 60° C. and 85° C., and most preferably between about 70°C. and 80° C. However, for some embodiments, the temperature does notneed to be raised to etch out the capillary. For example, in anembodiment where the sacrificial layer is HfN, the etch rate using thepreferred etchants is high enough that the whole capillary can be etchedwithout requiring additional heat. However, the etch process ispreferably facilitated by using a temperature within the ranges notedabove. A skilled practitioner can determine whether the material etchesfast enough down a narrow capillary, or whether heat should be applied.

As described below with respect to FIGS. 1A-1I, the etchants describedherein are useful in creating capacitors for integrated circuits. Anapplication describing the The H₂F₂-reduced etchant can remove asacrificial layer without substantially etching parts of the lowerelectrode or other surrounding layers. Because HF solutions with reducedH₂F₂ species remove sacrificial layers with minimal damage tosurrounding layers, they are particularly applicable to producingisolated capacitors.

FIGS. 1A-1I show an embodiment of a method for forming isolatedcapacitor structures using the reduced H₂F₂ species HF etchant. Astructural layer 10, preferably in the form of a first interlayerdielectric (ILD) layer, is deposited on a second ILD layer 20, as shownin FIG. 1A. In one embodiment, the ILD or structural layer 10 maycomprise an insulative material, such as borophosphosilicate glass(BPSG), or other generally known materials, such as phosphosilicateglass (PSG), tetraethylorthosilicate (TEOS), silicon, glass, polymer,etc. The second ILD layer 20 surrounds a conductive plug 25, whichpreferably contacts structural layer 10. The plug 25 preferablycomprises an electrical interconnect, which can connect the capacitor toanother circuit component, such as a transistor, resistor, etc. Althoughonly one plug 25 is shown in FIG. 1A, it will be understood that thereis a plug 25 for each cell in the array.

As used herein, “substrate” may comprise at least a portion of aconventional silicon wafer, but can more generally encompass structuressuch as semiconductive material or similar variations. Moreover, theterm “substrate” may also encompass any supporting or layeredstructures, including, but not limited to, oxide or semiconductivestructures, layers, or substrates.

In FIG. 1B, several containers or recesses 12 have been formed in thestructural layer 10. The recesses 12 can be formed through generallyknown mask and etch techniques, although preferably an anisotropic etchmethod, such as reactive ion etching (RIE) is used. Each recess 12 ispreferably etched to a depth of about 2 microns, preferably verticallyinto the structural layer 10. Each recess 12 is preferably etched toprovide an electrical contact to the conductive plug 25 and hassidewalls 14 and a base 16. Preferably, each recess 12 will beapproximately centered over one of the conductive plugs 25. The depth,shape and other properties of the recesses can be altered for differentapplications without departing from the scope of the present teachings.

Multiple recesses are shown in FIG. 1B and 1I to illustrate the memoryarray context for these capacitors. Preferably, the capacitors of theembodiment shown in FIGS 1A-1I are in a memory cell array. The recesseswill preferably be spaced apart between about 50 nm and 160 nm, morepreferably between about 60 nm and 150 nm. FIGS 1C-1H show only onerecess for simplicity of the drawings.

FIG. 1C illustrates the deposition of a sacrificial layer 30 on theupper surface 2 of the structural layer 10 and within the cell container12. Preferably, the sacrificial layer 30 will adhere to the base 16 andsidewalls 14 of the recess. In one embodiment, the sacrificial layer 30may be deposited using, for example, a known deposition process, such aschemical vapor deposition (CVD) or plasma enhanced CVD (PECVD). Thesacrificial layer 30 is preferably metallic and more preferably a metaloxide or a metal nitride. Some examples include AlN, Al₂O₃, HfN, andHfO₂. Many of these examples are dielectric materials, which hasconsequences for the sequence of steps for the process described herein.Most metal nitrides and metal oxides will be suitable materials for thesacrificial material, with the notable exception that titanium nitrideis not etched by the etchants described herein.

FIG. 1D illustrates the removal of the horizontal portions of thesacrificial layer from the upper surface 2 of the structural layer 10and the base 16 of the recess. This exposes the plug 25 for thedeposition of the lower electrode. This is preferably performed by adirectional etching process, more preferably a dry-etch anisotropicmethod, such as RIE. The directional etching process preferentiallyremoves horizontal surfaces in a process known as a spacer etch. Thedirectional etch leaves vertical layers, such as the sacrificial spacer30 on the sidewalls 14 of the recess.

FIG. 1E shows the formation of a lower electrode 40. The lower electrodeis formed by first depositing a layer within the recess and over thestructural layer 10, with the bottom of the electrode materialcontacting the plug 25. The lower electrode layer is preferably incontact with the sacrificial layer 30 on the sidewalls 14 of the recess12. The via is then filled with a protective filler (e.g., photoresist)and the structure subjected to chemical mechanical planarization (CMP)or other etch back process to leave the structure shown in FIG. 1E.

The lower electrode 40 should not be etched by the HF solution havinglow H₂F₂ species. In the embodiment shown in FIG. 1E-1H, the lowerelectrode is polysilicon, but there are also several other options forthe lower electrode materials. Preferable lower electrode materials alsoinclude titanium nitride and tungsten, but other materials may be usedwithout exceeding the scope of the present teachings. Preferably, thelower electrode is made of polysilicon or titanium nitride. Thesematerials are particularly well suited to acting as electrodes.

In the past, different etchants have been used to remove sacrificiallayers for different process flows with these different lower electrodematerials, because the etchants available were not selective to bothtitanium nitride and poly. However, with etchants of the presentteachings, specifically HF solutions having low H₂F₂ species, the sameetchant can be used for both lower electrode materials. Being able touse different lower electrode materials in different process flows withjust one etchant simplifies processing and allows for a more flexiblefabrication site without significant additional expense. Additionally,the excellent selectivity of these solutions from the sacrificialmaterial to doped and undoped oxides allows the sacrificial material tobe removed without substantially etching the field oxide.

It should be appreciated that the first conductive layer 40 may bedeposited using any one of a number of generally known depositiontechniques without departing from the scope of the present teachings.The thickness of the lower electrode can vary but preferably leaves roomfor subsequent conformal capacitor dielectric and upper electrodelayers. Preferably the lower electrode is between about 100 Å and 500 Åthick, more preferably between about 150 Å and 250 Å thick.

FIG. 1F illustrates the removal of the sacrificial spacers from thesidewalls 14 of the recesses 12. The removal of sacrificial layer allowsfor access to three surfaces of the structural layer 10 to assist insubsequently removing the structural layer 10. The removal of thesacrificial spacers is preferably performed by a HF solution having alow percentage of the H₂F₂ species of hydrogen fluoride. As discussedabove, the H₂F₂ species of hydrofluoric acid solution has been foundresponsible for most of the etching of doped and undoped oxides. Byusing an etchant according to the present teachings, excellentselectivity can be attained. For example, such an etchant willpreferably remove a metallic layer, such as AlN, Al₂O₃, HfN, or HfO₂,selective to poly with an etch rate selectivity above about 2000:1, morepreferably above about 10,000:1. The selectivity of metallic layers tooxide layers is preferably above about 500:1, more preferably aboveabout 800:1, most preferably above about 1,000:1. Even greaterselectivity can be attained with respect to tungsten and titaniumnitride, preferably etching metal oxides and metal nitrides at greaterthan 7,000:1 selectivity versus tungsten or titanium nitride. Withoptimization of the etchant to the conditions of the etching process andthe materials, the rate of selectivity can be even higher.

As shown in FIG. 1G, the structural layer 10 is removed before thedeposition of a conformal capacitor dielectric 50. The dielectric 50 isdeposited over the surface of the entire surface of the structure,including over the lower electrode 40 and the area formerly occupied bythe structural layer 10. Various materials can be used for the capacitordielectric 50, but some examples include some of the same materials thatare preferred for the sacrificial layer 30 (FIG. 1C), such as aluminumoxide (Al₂O₃) and hafnium oxide (HfO₂). In many high capacity DRAMs,tantalum pentoxide (Ta₂O₅) is used as the capacitor dielectric 50.Tantalum Pentoxide is easily deposited by CVD with good step coverage,and a high dielectric constant (“k value”) of between about 20 and 25.Many other insulators can also function as the capacitor dielectric 50.The thickness of the capacitor dielectric 50 is selected to leave roomfor the upper electrode, but the thickness can generally be betweenabout 10 nm and 350 nm, depending on the material and the capacitancedesired. Deposition of the capacitor dielectric 50 can be performedthrough well-known processes for deposition of similar dielectricmaterials, such as CVD, or deposition of a metal followed by oxidation.

The removal of the structural layer 10 (FIG. 1F) is facilitated by theremoval of the sacrificial spacers. The removal of the structural layer10, as shown in FIG. 1G, is preferably performed using a standard oxidewet etch. The capillaries where the spacer was located can be used toaccess the structural layer 10 from the sides, in addition to the top ofthe structural layer 10. Etching from three sides allows for quickremoval of the structural layer 10 and minimizes losses of theunpictured structural oxide in field regions outside the array.

FIG. 1H illustrates the deposition of an upper electrode 60, or secondconductive layer to form a double-sided container capacitor. In apreferred embodiment, the same material that was used for the lowerelectrode is used for the upper electrode. Other preferred materials forthe second conductive layer include tungsten silicide, elemental metal,or metal alloys. Skilled practitioners will appreciate that manyconductive materials can be used for the upper electrode. The upperelectrode's thickness can vary by several orders of magnitude dependingon the application, and it can also be made up of several thin layers ofconductive materials.

The capacitor in FIG. 1H can have several applications. Preferably, thecapacitor serves as a memory cell capacitor in a DRAM array. FIG. 1Iillustrates the capacitor of FIG. 1H adjacent another capacitor, aswould occur in a memory array. The distance between the double-sidedcontainers is preferably between about 50 nm and 160 nm, more preferablybetween about 60 nm and 150 nm. Additional layers can also be appliedover the capacitor illustrated in FIGS. 1A-1I. An example of this is aninsulator over the capacitor in order to further electrically isolatethe capacitor.

Structure

The capacitor is preferably in an array for use as memory cellcapacitors in DRAM. Individually, with reference to FIG. 1H, thecapacitor will have a lower electrode 40 that is not substantiallyetched by the etchant of the present teachings. Preferably the lowerelectrode 40 comprises polysilicon, tungsten, or titanium nitride. Thelower electrode 40 is preferably in direct contact with the plug 25. A.conformal capacitor dielectric 50 extends over the lower electrode 40.The upper electrode 60 extends over the conformal capacitor dielectric.Optionally, the capacitor is covered with a layer of an insulator, whichis not pictured, over the completed capacitors prior to uppermetallization layers.

The dimensions of the capacitor can relate to the performance of theDRAM memory cell array. With reference to FIG. 1I, the height of thecapacitor from the highest point 23 to base of the interior of thecontainer 21 is preferably between about 800 nm and 3000 nm, morepreferably between 1250 nm and 2500 nm. The diameter at the widest pointof the container capacitor is between about 100 nm and 450 nm, morepreferably about 175 nm and 375 nm. The distance 22 between twocapacitors in the memory cell array is preferably between about 50 nmand 160 nm, more preferably between about 60 nm and 150 nm.

The etch solution according to the present teachings can also be usedmore generally to selectively etch metallic layers, such as metalnitrides and metal oxides. In FIG. 2A, two oxide layers, 110 and 120 areon both sides of a metallic layer 100. In FIG. 2B, the metallic layer100 has been removed by applying the etchant of the present teachings.The etchant can comprise an ultradilute HF solution or an ultrabufferedHF solution as discussed above. As discussed above, the solutionsdescribed herein can also selectively etch metal oxides and metalnitrides relative to polysilicon and tungsten.

For etching some metal oxides in narrow spaces, heat is preferablyapplied to speed the etch process. As discussed above, some metal oxidesand metal nitrides etch faster than others do. For materials such asAl₂O₃, the etch rate may not be fast enough to reach the entirety of thesacrificial material. The heat increases the etch rate sufficiently toallow the entire sacrificial layer to be etched. An etchant according tothe present teachings is then used to remove the sacrificial spacer fromthe layered structure. The properties of the etchant allow for anexcellent selectivity ratio.

It will be appreciated by those skilled in the art that variousomissions, additions and modifications may be made to the methods andstructures described above without departing from the scope of theinvention. All such modifications and changes are intended to fallwithin the scope of the invention, as defined by the appended claims.

1. A method of forming an electrically isolated capacitor on a substratecomprising: forming a recess in a structural layer, where the recess hasa base and sidewalls; lining the recess with a metallic sacrificiallayer; forming a first conductive layer within the recess over thesacrificial layer; removing the sacrificial layer from the sidewalls ofthe recess using a hydrofluoric acid (HF) solution having a H₂F₂concentration of less than 0.06 mole fraction; depositing a conformalcapacitor dielectric over the lower electrode; and forming a secondconductive layer over the conformal capacitor dielectric.
 2. The methodof claim 1, wherein forming a recess comprises performing an anisotropicetching process on the structural layer.
 3. The method of claim 1,wherein lining the recess comprises a chemical vapor deposition process.4. The method of claim 1, wherein lining the recess comprises depositinga metal oxide or metal nitride.
 5. The method of claim 4, wherein liningthe recess comprises depositing a dielectric.
 6. The method of claim 1,wherein lining the recess comprises depositing aluminum nitride.
 7. Themethod of claim 1, wherein lining the recess comprises depositingaluminum oxide.
 8. The method of claim 1, wherein lining the recesscomprises depositing hafnium nitride.
 9. The method of claim 1, whereinlining the recess comprises depositing hafnium oxide.
 10. The method ofclaim 1, further comprising removing the sacrificial layer from the baseof the recess before forming the first conductive layer.
 11. The methodof claim 10, wherein removing the sacrificial layer from the base of therecess comprises performing a spacer etch process.
 12. The method ofclaim 1, wherein forming the first conductive layer comprises depositingtitanium nitride.
 13. The method of claim 1, wherein forming the firstconductive layer comprises depositing polysilicon.
 14. The method ofclaim 1, wherein forming the first conductive layer comprises depositingtungsten.
 15. The method of claim 1, wherein the removing thesacrificial layer from the sidewalls of the recess comprises using ahydrofluoric acid (HF) solution having a H₂F₂ concentration of less than0.04 mole fraction.
 16. The method of claim 1, wherein removing thesacrificial layer comprises using a HF solution which has a ratio ofwater to HF of between about 1,000:1 to 10,000:1 by volume.
 17. Themethod of claim 16, wherein removing the sacrificial layer comprisesusing a HF solution which has a ratio of water to HF of between about1500:1 to 5000:1 by volume.
 18. The method of claim 16, wherein removingthe sacrificial layer comprises using a HF solution which has a ratio ofwater to HF of between about 2500:1 to 4000:1 by volume.
 19. The methodof claim 1, wherein removing the sacrificial layer comprises using a HFsolution which has a pH of between about 3.5 to 5.5.
 20. The method ofclaim 19, wherein removing the sacrificial layer comprises using a HFsolution which has a pH of between about 4.0 to 5.0.
 21. The method ofclaim 19, wherein removing the sacrificial layer comprises using a HFsolution which has a pH of between about 4.25 to 4.75.
 22. The method ofclaim 1, wherein depositing the second conductive layer comprisesdepositing the same material as the first conductive layer.